1. Field of the Invention
This invention relates to an apparatus for and a method of aligning a plurality of bodies.
2. Description of the Prior Art
In the manufacture of semiconductor integrated circuits, the steps of aligning a mask and a wafer and subsequently transferring the circuit pattern of the mask onto the wafer are repetitively effected. Alignment of the mask and the wafer is realized by registering alignment marks with each other. Since various patterns are transferred onto one wafer with masks interchanged sequentially, alignment marks for several steps are provided on the same wafer in advance.
To align the mask and the wafer, it is known to provide two or three alignment marks on the mask and wafer, respectively, scan these alignment marks by an emitted energy beam such as a laser beam, receive the energy scattered by the alignment marks and convert it into an electrical signal. The alignment marks and signal detecting apparatus of this type are described in U.S. Pat. No. 4,167,677, etc.
There is also known a system in which the images of alignment marks are picked up by a video image pickup tube or a photosensor array and the signals thereof are processed to obtain an alignment signal.
The prior art apparatus for aligning a mask and a wafer in the manufacture of semiconductors is constructed as shown, for example, in FIG. 1 of the accompanying drawings. The alignment marks M and W on a mask 4 and on a wafer 6 placed on a stage 5 are scanned by a laser beam L emitted from a laser light source 1, through a polygonal mirror 2 and a beam splitter 3, and the scattered light from the alignment marks M and W are detected by a photoelectric detector 8 through a condenser lens 7. The detected signals are supplied to a control circuit 9, which calculates the amount of displacement of the mask 4 and wafer 6 from these signals and moves the wafer 6 by drive motors 10, 11 for driving the stage 5 in X- and Y-directions and a drive motor 12 for driving the stage 5 in the direction of rotation, thereby accomplishing the aligning of the alignment marks M and W. Generally, however, the laser beam L scans also the portions on the wafer 6, for example, other than the alignment mark W in the direction of arrow a, as shown in FIG. 2 of the accompanying drawings, and therefore, a synchronizing signal for taking out the signal only on the alignment mark W, namely, a so-called window signal, is required. For this, for example, a small aperture 14 formed in a disc 13 rotatable about the same axis as the polygonal mirror 2 of FIG. 1 is detected by means of a photosensor 15, and, from the signal S.sub.1 from this photosensor 15 shown at (b) in FIG. 2, a certain time signal, i.e., a prewindow signal S.sub.2 shown at (c) in FIG. 2, is obtained and, further therefrom, a window signal S.sub.3 corresponding to the time b during which the laser beam L scans the alignment mark W is provided. However, during the setting prior to the aligning, the wafer 6 is usually displaced as indicated by 6b or 6c in X- and Y-directions relative to the aligned position 6a, as shown at (a) in FIG. 3. Thus, the position of the alignment mark W is displaced as indicated by Wb or Wc relative to the scanning of the laser beam L in the direction of the X-axis. Therefore, the window signal S.sub.3 must have a width c great enough to permit scanning of the alignment mark W even when the position of the wafer 6 has been displaced at maximum in the X-direction. Also, blank areas of widths d and e, respectively, are required on the left and right of the alignment mark W so that no other signal than the signal on the alignment mark W may be deleted within the duration of the window signal S.sub.3 at this time.
For example, where alignment marks M as shown at (a) in FIG. 4 of the accompanying drawings are placed on the mask 4 and alignment marks W as shown at (b) in FIG. 4 are placed on the wafer 6 and an attempt is made to align these into the aligned state as shown at (c) in FIG. 4, it is assumed that they are initially in the positional relation as shown at (a) in FIG. 5 of the accompanying drawings. At this time, by the scanning of the laser beam L, a pulse train as shown at (b) in FIG. 5 is obtained in the output of the detector 8. Characteristics are extracted from the spacings in this pulse train on the basis of the reference width f of the mark M on the mask 4, and the amount of displacement of the mask 4 and wafer 6 is found out. However, if a signal g.sub.1 on the adjacent alignment mark W' of the wafer 6 for the next step exposure, for example, comes into the duration of the window signal S.sub.3 such as shown at (c) in FIG. 5, the signal g.sub.1 cannot be distinguished from the effective signal and aligning cannot be accomplished. On the other hand, the window signal S.sub.3 need have a width c large enough to cover a signal g.sub.2 of the most distant mark W.sub.1 even where the wafer 6 when set has been displaced at maximum. Further, if any borderline h such as by a step exists between adjacent alignment marks where a plurality of sets of alignment marks W are juxtaposed in the direction of the scanning axis as shown at (a) in FIG. 6 of the accompanying drawings, the laser beam L is scattered at that position and enters the detector 8 intact. If this occurs within the duration of the window signal S.sub.3, it cannot be discriminated from the signals of the marks W and aligning cannot be accomplished. Accordingly, sufficient blank areas need be provided on the left and right of each mark M so that the signal of the adjacent mark or the borderline h may occur within the duration of the window signal S.sub.3 even if the spacing i.sub.1 between the marks W is displaced at maximum during the setting where there is no borderline between the adjacent alignment marks W of the wafer 6 or even if the spacing i.sub.2 between the end of the mark W and the borderline h is displaced at maximum during the setting where there is a borderline h between the adjacent alignment marks W of the wafer 6. In the case of alignment marks M and W having an inclination of 45.degree. relative to the direction of the scanning axis a of the laser beam L as shown in FIG. 4 and when the wafer 6 is displaced by .DELTA..sub.y in the Y-direction as shown at (b) in FIG. 6 of the accompanying drawings, it is also displaced by .DELTA..sub.y in the X-direction which is the direction of the scanning axis. Thus, actually, it is also necessary to take into consideration, the maximum amount of displacement of the wafer 6 in Y-direction. Where, as shown in FIG. 7 of the accompanying drawings, the spacing on the scanning axis a between the alignment marks W of the wafer 6 in their aligned state is j and the maximum amount of displacement thereof in the X-direction is .DELTA..sub.x and the maximum amount of displacement thereof in the Y-direction is .DELTA..sub.y, the width c of the window signal S.sub.3 need be j+2(.DELTA..sub.x +.DELTA..sub.y). If the distance between the marks W is k and the distance l is suitably determined, in a case where there is no borderline between the marks W of the wafer 6, the necessary condition is EQU k.ltoreq.j+2(.DELTA..sub.x +.DELTA..sub.y +l) (1),
and in a case where there is a borderline between the marks W of the wafer 6, the necessary condition is EQU k&gt;j+2(2.DELTA..sub.x +.DELTA..sub.y +l) (2)
Thus, the size of the blank areas in the direction of the scanning axis of the alignment marks W of the wafer 6 greatly depends on the amount of displacement ".DELTA..sub.x and .DELTA..sub.y " of the wafer 6 before aligned, and it is impossible to make the areas of the alignment marks W smaller than the dimensions prescribed by formulas (1) and (2) above.